Random access memory integrated circuits typically contain millions of essentially identical cells integrated on a single-crystal silicon substrate, each cell capable of storing one bit of digital information. Dynamic random access memory (DRAM) cells are popular because of their relatively simple cell structure, usually consisting of a single capacitor storage element coupled to an access transistor. However, DRAM memory is volatile and must be frequently refreshed to maintain data integrity. Also, reading the data stored in a DRAM cell is a destructive process, such that the data must be rewritten each time it is read. A further problem with the basic DRAM structure is the inherent difficulty in scaling the memory cell to smaller size, since the charge storage requirements for the cell capacitor do not scale proportionally. As a result, alternative materials and structures are now receiving serious consideration for memory applications.
Ferroelectric random access memory (FRAM) structures make use of the remanent polarization properties of a ferroelectric material to store data. FRAMs may generally be divided into two types, depending on whether a ferroelectric capacitor or a ferroelectric transistor is used as the storage element in a memory cell. A capacitor-based FRAM is similar to a DRAM in operation and basic layout, and while it may have the advantage of non-volatile data storage, it still has a destructive readout and a scaling problem. In contrast, transistor-based FRAM cells are generally more complex, but in theory offer higher performance; such cells have been proposed with both non-volatile storage and non-destructive readout features. Unfortunately, researchers have been largely unsuccessful in their attempts to deposit ferroelectric thin films as gate dielectrics directly upon silicon transistors, a necessary step for commercial development of these basic transistor FRAMs.
An alternative FRAM structure is disclosed by Evans et al. in U.S. Pat. No. 5,119,329, issued Jun. 2, 1992, which avoids interfacing ferroelectric material directly with a silicon substrate by using a thin film semiconductor overlying a ferroelectric thin film as the storage device. The thin film semiconductor operates as a variable resistor with resistance set by the polarization state of the ferroelectric material, such that data may be read from the device by sensing the resistance of the thin film semiconductor which overlies the ferroelectric material.